1. Field of the Invention
The present invention relates to a system and method for generating a transmission digital signal from a source digital signal and reproducing the source digital signal as a reproduced digital signal from the transmission digital signal, and in particular, to a system and method for suppressing an electromagnetic noise generated by the transmission digital signal. The source digital signal is, for example, a clock signal and a data signal.
2. Prior Art
Since a clock signal is composed of repetitive pulses having the same frequencies, the clock signal has a large spectrum at the clock frequency and its higher harmonic frequencies. When the clock signal is transmitted through a transmission path, the signal radiates electromagnetic noise having these frequencies. A data signal does not have a higher periodicity than the clock signal. However, a video signal and an audio signal which have been band-compressed have a high entropy. Thus, these signals have a high periodicity and large spectrums at ½ frequency of the clock signal and its higher harmonic frequencies. Therefore, a data signal transmitted through a transmission path radiates electromagnetic noise at these frequencies.
Conventionally, in order to suppress the electromagnetic noise, the voltage amplitudes or current amplitudes of a clock signal and a data signal are decreased. In addition, a differential transmission system such as ECL (Emitter-Coupled Logic) is used.
As a prior art reference, a method and apparatus for suppressing the electromagnetic noise are disclosed in JPA 9-289527. According to the prior art reference, a clock signal is frequency-modulated. The frequency of a spectrum generated from the clock signal is dispersed so as to suppress the electromagnetic noise.
In the method and apparatus of the prior art reference disclosed as JPA 9-289527, a source clock signal is frequency-modulated with a particular frequency so that the frequency of the transmission clock signal is dynamically varied. On the reception side, a PLL removes a modulation signal component so that the source clock signal is reproduced. The frequency change ratio of the transmission clock signal which has been frequency-modulated is supposed to be±several percent.
Generally, the lock range and capture range of a PLL are as high as several hundred ppm at the maximum. Thus, in the method and apparatus of the prior art reference, the conventional PLL circuit cannot be used to perform adjustment in a several percent range. Instead, a more complicated circuit is required.